Benoit, M.

Dubois, Jacques

Horizon / Pleins textes

The Earthquake Swarm in the New Hebrides Archipelago, 
Aal,agnsit 896s 
FIG. I.-Uplifted coastline on the north-west coast of Mallicolo. The white markings are caused by dead coral raised 
above hish-tide level. 
FIG. ?.--Closer view of the uplifted coastline. The amount of uplift was appro-$mately 80cm. i @ 
p Xcce!Q. Ciiiltol M o i . e ~ g t i t s .  Royal Society of iVezo Zeulaiirl, Bulletin 9, p p .  141-148;"9  fi?^. 1971 _ _  Fonds Documentaire QRSTOM 
i42 RECENT CRUSTAL MOVEMENTS 
MAC ROSE IS MI^ OBSERVATIONS 
The earthquakes of 1965 August l l d  03h 40m 56s, 
I l d  2% 31m 5% and 12d 08h Olm 43s U.T. were 
responsible for the most serious damage. 
1. Collapse of the wharf at Port-Sandwich and of 
the Quai Gubbay at Luganville. 
2. 
example, at Lugmville, Norsup, Sarmette, Lah. . 
toro, and Lamap. 
3. Breakage of water supplies and telephone lines. 
On August 13 an aftershock gave rise to a tsunami 
2m in height, which resulted in the loss of seved 
small ships on the west of Espiritu Santo, and 
destruction of coastal installations. 
i 
d 
-1 Serious damage to buildings in many places, for 
'2 
'<&- 
Y 
168O 
o 
U 
1 8 O  S 
1 
1 8 O  S 
Earthquake Sicnrm in t h  N e w  Hebrides-BExorT AND DUBOIS 
- Saïsme prémonitoire - 
H o  s 03. LO. 56. 
M = 7 -  N O U  
143 
O E  
FIG. 4.-Isoseismals of the earthquake of 1965 August l l d  03h 40m U.T. 
-Seisme principal - 
H o =  22 . 37 .  S L .  
M = 7 , 7 - ' N O U -  
1 6 6 O  E 
FIG. 5.-Isoseismals of the earthquake of 1965 August l l d  08h Olm U.T. 
::< 
$7 On August 11 and 12, an uplift of 0.5-0.8m in the 
$$. northern part of the island of Mallicolo appeared 
,k 
$2 along lOOkm of the coastline, and in the sea bed in *' the Norsup region (Figs. 1 and 2 ) .  In addition, !: slumping of the ground, landslides, and fissuring were 
< .  found on all the islands inspected. Figs. 4 and 5 
are isoseismal maps for the two largest shocks, on 
_, August 11. Some of the aftershocks resulted in less 
serious damage. There were 62 on August 11, 12, 
and 13. 
EPICENTRE DETERMINATIONS 
Epicentre determinations were made by the follow- 
ing methods, using the five stations of the ORSTOM 
network, at Noumea, Koumac, Port-Vila, Ouanaham, 
and L u g a n d e  (Fig. 3 ) .  
7 
? 
P 
M z. 
:: 
I 
Y 
W 
vl 
m 
09 
E 
R 
O vl
3 
NOUMEA 
Bulletin n a  31 
d u  24 AÔut 19GS 
15"t Sud 
166O8 Est 
Rr: 33Km.  
HO= 0.3. 4 0 .  5 6  
Magnitude : 7 .  
I 
166O EST I 167"  168O 
c 
Epr'centr e 
U.S.C.G.S. 
/ 
LUG. 
--1S0 S U D  
;a 
m 
M n 
i! 
-I 
-- 16' 
U.S.C. G .  S .  
Lu g anvilla 
N o r su.p 
Paama 
Epi 
Port - Vila 
P D  E n e  6 6 .  Aug.  27. 1965 
/ 
l S b  45 Sud f 3,3 Km. 
166O91 Est f 3,s Km. 
h =  2 6 K m .  2 5,3 Km. 
Ho= 0 3 .  L O .  5 6 , Z f  0,75. 
Nbrc de stations = L9 
S D.= 1,2 
Magnituda = 6,3 -7 Stations 
Earthquake Swarm in the Nezu Hebrides-BENOIT AND DUBOIS 
167* E 167 
Epicentres Hagni tu  de 
NOUMEA DATE 
e <5 
a 
de 5 à < 6  
de 6 à<7 
7 et plus 
-LEG E N DE- 
Ho.22.3/.5A 
11-8-65 .  E S P I R I T U  
SANTO 
I 
*=S6 I I -  8 -  65. 
Pic Sonto \ *  
4 
e l 
e 
\, 
\ 
\ *  
\ 
AOBA 
PEN 
ANorsup 
R 
\ 
e 
\ w;. Lamap S ndwi, 
o 
O 
F MALLIKOLO 0 . 0  
\ 
\ 
\ 
r 
I 
FIG. 7.-Epicentres of earthquakes occurrine 
F r o m  P-arrival Times.-Graphs were constructed 
showing the foci of epicentres giving a constant 
difference in, arrival time at a pair of stations. These 
were determined point by point, as the apparent 
velocities of propagation (obtained from the 1940 
Jeffreys-Bullen Tables) are dependent upon the epi- 
central distance. 
A circle of radius nkm centred at  one of the stations 
will correspond to a travel time t l .  The positions 
of the epicentres corresponding to differences of 
145 
Dirrdions Tectonique 
majeures 
4-460 S o u l h m e n t  dr 
ta cotc en mitras u MAEVO 
P A A M A  
.?o 
COPEVI 
E P I  
E M A H E  
' during the 1965 New Hebrides swarm. 
arrival time of 1, 2, 3, . . , nt seconds will lie at 
the intersection of the first circle with a family of 
circles centred at the second station, and having 
radii corresponding to the travel times tl + 1, 
tl + 2, tl + 3 . . . t l  + 111 seconds, as given by 
the tables. 
In  this way, the seismic region of the New Hebrides 
was covered by a set of 10 graphs (corresponding to 
five stations, or 10 station-pairs). and worked out for 
different focal depths. 
146 RECENT CRUSTAL MOVEMENTS 
I 
I 
I 
I 
A -  
I I  I 
o I o 
\ I m  - , Y  
O n o  œ O I 
4 I c 
I 
I 
I I I 
0 U r  O r N m 
'I I +  I l 
N U -  O c 
O 
I C  
O 
I '  
2 x +I  
I 
CI 
t 
r 
I 
Mag. 
U.S.C.G.S 
D a t e  
Energie parjolur 
en Ergs  
U.S.C.G.S. 
E n e r i e  des 3 s e ï s m e s  
principeaux en 7. 
d e  L’dnergie totalc’ 
9 1  7. 
11 Aout 
2,3 102’ 
12 15 16 17 18 19 2 0  21 
5,3 101~ 3,i 1019 :,i 1017 
11 
I 
3,3 101~ Ergs  Energie U.S.C.G.S. 
- 
totale par j o u r  70,o 26,s 1,6 1,6 0>9 0,o o, O0 
70 de l’énergie 
U.S .C.G. S.. 
FIG. 9.-Accuinulatcd cncrsy rclcase bctwc‘cn 11 and 25 
August 1965. 
- 
RECENT CRUSTA 
Epicentral Distances.--When S-waves were 
corded, the S-P intervals could be used to 
epicentral distances from the Jeffreys-Bullen 
Tables. Fig. 6 shows an example of an epicentre 
determinati&, for the shock of .&ut I ld  03h 05, 
56s. During the swarm: 21 1 such determinations were 
made. 
In  its bulletins, the USCGS prints epicentres based 
on the records of its world-network stations for most 
of the shocks. The USCGS positions seem to be 
systematically displaced a little to tkie east \vith respect 
to ours, which are in good agreement with the macro- 
seismic observations of the main shocks (Figs. 4 and 
5)  . 'The principal epicentres determined at Nounlea 
are shoivn in Fig. 7. 
HISTOGRAMS OF USCGS TIME-RESIDUALS 
In, the USCGS Earthquake Data Reports, O-C 
residuals (observed-calculated arrival times) are given 
for the stations used in their epicentre determinations, 
and these include Noumea. Histograms of these 
residuals have been drawn for the stations at Noumea, 
Kouinac, and Port-Vila, which recorded the greatest 
number of shocks. Luganville was put out of action 
by the first shock, and Ouanaham recorded only a 
small number of aftershocks, because of its low 
qiagnification. The variances are close to 1, which 
corresponds to a probable error of the average values 
that is less than k0 .2  seconds. 
I t  is concluded that the tlvo New Caledonian 
stations have identical average values of O-C = 
-1 r+ 0.2 seconds, and that the corresponding value 
for Port-Vila is -0.1 -i 0.1 seconds. Further, O-C 
at Nouniea and Koumac increased from -2.5 to 
about 0.5 seconds during the first three days of the 
swarm (see Fig. 8) . The regression lines obtained are : 
Nouniea (O-C) = 0.0262 tl, -L 2.306 
Koumac (04) = 0.0159 ta - 1.683 
This result will be used in the interpretation. 
ENERGY RELATIONS 
Values of unified magnitude M are quoted in the 
USCGS bulletins. The liberated energy in ergs can 
be obtained from these by the relation 
log E 5.8 + 2.4 M (Richter, 1958). 
Fig. 9 shows the magnitudes M as a function of time, 
a table of liberated energies, and a graph of the 
accumulated energy release. Of the total energy, 
95 percent was liberated in the three niain shocks, 
45 percent of it in the shock of August l l d  22h 
31m 54s. 
INTERPRETATION 
Although the epicentres are accurately known, the 
focal depths are uncertain by & 20km at least. This 
is apparent from several of the observed facts: 
I .  The increase in the value of O-C at Nouiiica 
and Kouniac could be the result of a decrease 
in, focal depth. C remains constant, since 
Jeffreys-Bullen Tables for a uniform depth of 
33km were employed throughout, and O 
increases. 
2. The aftershocks recorded after the iliain events 
show the multiple P and S pulses characteriscic 
of crustal earthquakes, and both surface-waves 
and T-waves are prominent. 
3. There was no tsunami associated with two of 
the main shocks, although their epicentres were 
at sea; but the small aftershock of August 13 
produced a tsunami 2 metres high. 
These obsen-ations can all be explained if the 
three main, earthquakes, originating beneath the crust, 
are considered to have given rise to crustal readjust- 
ments, in which only 5 percent of the total energy 
was liberated. These aftershocks were, moreover, 
for the most part located in the east-west fracture 
zone (Fig. 7 )  reported by geologists and readily 
visible in aerial photographs, which corresponds to 
the southern limit of the north Mallicolo zone of 
uplift. 
REFERENCES 
JEFFRE&; H.; BULLEN, K. E., 1968. Seismological Tables. 
RICHTER, C. F., 1958. Elementarj1 Seismology. Freeman 
, .  
Gr;ry-Milne Trust, London. 50 pp. 
and Co., San Francisco. pp. 364-367. 
c 


Recent crustal movements

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